Wireless communications systems have become ubiquitous. With so many systems emitting radio frequency energy, the environment is filled with energy that can interfere with systems that are trying to communicate. Many wireless systems such as wireless local area networks, cordless telephones and Bluetooth® compatible devices transmit a low power signal where interfering signals often have energy levels that are magnitudes greater than low power systems. Accordingly, it can be difficult to communicate low power digital data with such high levels of interference.
Many wireless communication systems utilize a wideband topology. Wideband receivers can simultaneously transmit and receive on numerous frequencies within a given bandwidth. Many wideband receivers, such as those utilized by wireless local area networks (WLANs), can be subjected to narrow band interference from other devices that emit interference having frequencies that fall within the pass band of the wideband receiver. Some interfering devices may not even be communication systems. For example, microwave ovens and motors can emit interference.
To address such interference there has been significant effort in the field of noise mitigation. One such interference mitigation technique is noise cancellation, where disruptive noise can be detected and inverted (shifted 180 degrees) to create an anti-phase signal that can be added to a delayed incoming signal. Such a process can cancel at least a portion of the noise component, making it easier for a system to recover data over a wireless link. Traditional systems often have multiple receive paths, one path for the desired signal and another path for generating the anti-phase signal. Thus, wideband receivers that utilize four channels often have eight receive paths. Other traditional interference mitigation systems utilize filtering processes. These filtering processes are generally not effective for mitigating interferences that have frequency that fall within the receiver pass band because filtering these frequencies degrades the desired signal.
Narrowband interference is a type of electromagnetic interference that often occurs at relatively high levels in a band of frequencies that are smaller or narrower than the total bandwidth of the receiver experiencing interference. One common type of narrow band interference that is problematic for WLAN type devices comes from radios transmitting voice or data at low rates. A few examples of such narrow band type devices can include cell phones such as global system for mobile communications (GSM) phones, Bluetooth® compatible devices and cordless phones.
Generally, TDMA (time division multiple access) type devices are allocated a time slot in which they can transmit and receive, and thus TDMA devices create periodic interference or high level burst interference that can be disruptive for systems that operate at low signaling levels. Although such devices are typically assigned an “exclusive” frequency band, sidebands or harmonics of the transmitted frequency that are emitted often fall within the active bandwidth for other systems.
One current standard for the wideband wireless local area networks is the Institute of Electrical and Electronic Engineers (IEEE) 802.11 standard, originally published in October of 1999. The 802.11 standard specifies that WLAN device enter a power conservation mode (and not transmit or receive) when the signal being received is less than a predetermined amount over the noise level. This standard caters to battery powered WLAN compatible devices because if no signal is available for reception, the system can enter the sleep mode to conserve battery power. High levels of interference can mask the desired signal so that the WLAN receiver is unable to correctly operate in the power conservation mode.
While filtering techniques can be used to remove certain types of interference, filtering can also cause a loss of data, since filtering typically degrades the signal or removes part of the desired signal. Some have attempted to utilize a tap notch finite impulse response (FIR) filter to mitigate interference though filtering. This approach is not very practical because the length of the notch filter (required sampling size) is typically too large, and such a filter requires more than a WLAN preamble length for initialization. For example, a typically WLAN compatible transmission has a preamble with 10 standard transfer specification (STS) which requires 160 samples. The overall latency created by such a FIR filter is too high, and the complexity of such a system is also a limiting factor.
As stated above, the feature in the IEEE 802.11 specification that is intended to reduce power consumption of a WLAN receiver when a useable signal is not present often causes a dropped communication. In addition, 802.11 compliant systems can easily be jammed with an interfering signal even though proper interference mitigation would allow an 802.11 compliant signal to be effectively processed by the receiver. Traditional receiving systems without robust interference mitigation systems often have communication failures, even in the middle of packet reception. Thus, most traditional receiving systems have less than perfect interference mitigation systems, and such systems will drop a communication session even though with proper mitigation a useable signal would be present and communications could continue uninterrupted.